A discharge lamp that uses the luminescence of a fluorescent material such as a fluorescent lamp is basically a construction that causes a glow discharge inside an airtight discharge space in which a low-pressure gas has been sealed, converts the ultraviolet rays that are emitted by this glow discharge to visible light by means of a fluorescent film that is provided on the inner walls of an enclosure, and extracts this visible light to the outside from the translucent portion of the enclosure. One such discharge lamp is a variable-color discharge lamp that is capable of switching the color of the output light of a single discharge lamp among a plurality of colors. The color of output light of a discharge lamp is chiefly determined by the wavelength of the ultraviolet rays that are generated from the glow discharge and by the fluorescent material that is excited by these ultraviolet rays. If the variable-color discharge lamps of the prior art are considered in terms of the sealed gas, which affects the wavelength of the ultraviolet rays that are generated by the glow discharge, the fluorescent material, the electrode structure that relates to the switching of the output light color, and the lighting method, the types of variable-color discharge lamps that are known thus far are as follows:
First to be considered are variable-color discharge lamps that use a plurality of fluorescent materials, each having a different emitted color. The plurality of fluorescent materials are in some cases used in mixtures and in others stacked in a layered structure. Alternatively, the materials are individually formed in separate locations inside the discharge space.
As an example, Japanese Patent Laid-Open Publication No. 2001-266801 discloses a variable-color discharge lamp that uses a mixture of two types of fluorescent materials, a fluorescent material for mercury light emission and a fluorescent material for xenon light emission, and further, that uses a gas mixture of mercury vapor and xenon gas for the discharge gas (Prior Art Example 1). In this discharge lamp, making the waveform of the voltage that is applied between electrodes a sine wave tends to excite the mercury while making the waveform a pulse wave tends to excite the xenon, and the color of the output light can be switched by changing the rate at which the two types of fluorescent materials are excited. This variable-color discharge lamp employs two types of fluorescent materials and a gas mixture of two types of gases and includes one pair of electrodes. The position of formation of the discharge does not vary within the lamp.
In the same publication, prior art is also disclosed in which the color of emitted light that is obtained outside a lamp is varied by: employing a container having a double-layer structure that includes an outer tube and an inner tube that is inserted inside the outer tube, applying a fluorescent material that emits red light to the inside of the outer tube and applying a fluorescent material that emits green light to the inside of the inner tube, and then making the waveform of the voltage that is applied between the electrodes a sine wave or a pulse wave to switch between generating a positive column of discharge between the outer tube and inner tube or generating a positive column inside the inner tube (Prior Art Example 2).
This variable-color discharge lamp is similar to the first example of the prior art with regard to the use of two types of fluorescent materials and a single pair of electrodes, but differs regarding the use of xenon as a single sealed gas. In addition, the principal of the operation of this variable-color discharge lamp further differs in that the two types of fluorescent materials are separately provided at separate locations inside the discharge enclosure, and in that, through the design of the electrode construction, the region in which the positive column of discharge forms within the discharge enclosure is changed in accordance with the waveform of the applied voltage. One of the pair of electrodes of this discharge lamp is an inner electrode similar to the first example of the prior art, and the other electrode is an outer surface electrode that is constituted by a thin threadlike conductor that is wound in a spiral around the outside of the outer tube.
Japanese Patent Laid-Open Publication No. H07-085843 similarly discloses a variable-color discharge lamp that employs two types of fluorescent materials that emit light of different colors (Prior Art Example 3). This discharge lamp employs xenon as a single sealed gas and a pair of inner electrodes similar to the electrodes in the first example of the prior art, but varies the rate of contribution to the emitted light made by each of the fluorescent materials by varying the crest value of the pulse voltage that is applied between the electrodes.
Japanese Patent Laid-Open Publication No. H06-076801 discloses a variable-color discharge lamp that, as in the third example of the prior art, employs two types of fluorescent materials that are excited by ultraviolet rays of different wavelengths and that varies the rate of contribution to the emitted light made by each fluorescent material by varying the conditions of the voltage that is applied between the electrodes (Prior Art Example 4). In the discharge lamp according to this fourth example of the prior art, varying the duty ratio of applied pulses causes a change in the distribution of the wavelengths of the ultraviolet rays that are emitted by the mercury of the sealed gas and thus changes the light-emitting intensity of each of the fluorescent materials.
Variable-color discharge lamps that employ only one type of fluorescent material are also known. The above-described first to fourth examples of the prior art employed a plurality of types of fluorescent materials that each emits light of a different color, but Japanese Patent Laid-Open Publication No. H07-029549, Japanese Patent Laid-Open Publication No. H06-310099, and Japanese Patent Laid-Open Publication No. H07-006734 each disclose discharge lamps that are capable of varying the color of emitted light while employing only one type of fluorescent material (Prior Art Example 5). These discharge lamps are all similar in that they each employ two pairs of electrodes and a gas mixture of two types of gas as the sealed gas, and further, in that they vary the color of emitted light by switching the electrode pair that causes discharge.
For example, the variable-color discharge lamp that is described in Japanese Patent Laid-Open Publication No. H06-310099 has a pair of inner electrodes inside and at both ends in the longitudinal direction of a straight-tube bulb. In addition to these inner electrodes, the lamp is further provided with a pair of outer-surface electrodes on the outer surface of the bulb. Further, a gas mixture of two types of gas such as mercury and neon that emit ultraviolet rays of different wavelengths is sealed inside the bulb. In this discharge lamp, the application of a high-frequency voltage between the inner electrodes causes mercury vapor to be ionized and excited in a positive column that is generated between the inner electrodes to produce ultraviolet rays, and these ultraviolet rays excite the fluorescent material to produce visible light of a color that accords with the characteristics of the fluorescent material. When high-frequency power is applied between the outer-surface electrodes, on the other hand, a glow discharge is generated by the dielectric barrier discharge between the outer-surface electrodes, neon is ionized and excited in the portion of this negative glow, and visible light of the red color peculiar to neon is generated and extracted to the outside.
Japanese Patent Laid-Open Publication No. H10-003887 discloses a noble-gas discharge lamp (Prior Art Example 6) that uses two pairs of electrodes as in the fifth example of the prior art and that is further capable of toning the color of the emitted light over a wide range by switching the electrode pairs that are caused to discharge. In contrast to the above-described first to fifth examples of the prior art, this discharge lamp is a flat noble-gas discharge lamp. In this noble-gas discharge lamp, a plurality of first electrodes and second electrodes that are in an electrically insulated state from the first electrodes are alternately arranged on the inner walls on the discharge-space side of a rear-surface substrate having a flat shape. Third electrodes having a size that corresponds to the entire region in which the first and second electrodes are arranged on the rear-surface substrate are provided on the outer surface of the light-extraction side of the substrate that confronts the rear-surface substrate. A first fluorescent material film is provided over the first electrodes of the rear-surface substrate, a second fluorescent material film that emits light of a different color than the first fluorescent material film is provided over the second electrodes, and the discharge space is charged with the single gas xenon.
In the flat noble-gas discharge lamp of this sixth example of the prior art, a lighting operation in which a high-frequency voltage is applied between the first electrodes of the rear-surface substrate and the third electrodes of the light-extraction side substrate and a lighting operation in which high-frequency voltage is applied between the second electrodes of the rear-surface substrate and the third electrodes of the light-extraction-side substrate are executed in time divisions. The color of the emitted light can then be toned over a wide range by controlling such factors as the proportion of the lengths of the intervals of carrying out each of the lighting operations and the frequency and voltage value of the voltage applied in each lighting operation.
In contrast to the discharge lamps described in the above-described first to fifth examples of the prior art, which all used straight-tube bulbs, the construction of this noble-gas discharge lamp of the sixth example of the prior art differs significantly because it is a flat noble-gas discharge lamp that employs a bulb of flat-panel construction. As previously described, a variety of a variable-color discharge lamps are known in the prior art that are each capable of varying the color of emitted light over a plurality of colors using a single discharge lamp, but the flat noble-gas discharge lamp of the sixth example of the prior art in particular has a flat bulb and is therefore better suited for obtaining a thin-surface light source than the variable-color discharge lamps of the first to fifth examples of the prior art that employ cylindrical straight-tube bulbs. Moreover, the flat noble-gas discharge lamp of the sixth example of the prior art is further capable of widely varying the color of emitted light to an extent that is virtually free of stepped gradations.
However, in the flat noble-gas discharge lamp of the sixth example of the prior art, two types of fluorescent materials that produce emitted light of different colors must be applied in prescribed patterns to each of the rear-surface substrate and light-extraction-side substrate at each time, and the fabrication steps are therefore complex and fabrication is correspondingly difficult. In addition, two power supply devices must be operated in time divisions to realize color toning, the operating intervals of each of the power supply devices when carrying out the time-division operation, i.e., the lighting interval by the first electrode and third electrode and the lighting interval by the second electrode and the third electrode, must be switched in time intervals sufficiently short to prevent flicker that is noticeable to the eye, and the frequency and voltage level of the output voltages of the power supply devices are also changed in each lighting interval, and due to all of these factors, the power supply devices and lighting control are inevitably complex. Thus, although the flat noble-gas discharge lamp of the sixth example of the prior art enables switching of the color of the emitted light with dramatic diversity, this diversity comes at the cost of the above-described side effects.
In contrast, when a flat noble-gas discharge lamp is used as the light source of an illumination device, it can be assumed that the discharge lamp need only supply two and at the very most four colors of emitted light, that the lamp need only allow switching and has no need for color toning that is free of stepped gradations, and further that the lamp be of simple construction that does not entail complex fabrication procedures. For example, switching the color of the emitted light of an illumination device to a daytime color in the morning and during the day, to a light-bulb color at night, to a color tinged with blue that provides a strong sense of coolness during the summer, and then to a color tinged with warm red during the winter can produce a desired atmosphere by switching the illumination color to a color that is appropriate to the season and time of day. When a discharge lamp is used for such a purpose, there is no need to allow switching of the illumination color over such a wide range of illumination colors. In addition, switching of the power supply device can be adequately realized by a construction that allows manual operation of a mechanical switch.